Regulation of gene expression in the developing immune system is studied. One goal is to elucidate the mechanism of transcriptional regulation of major histocompatibility complex (MHC) class I genes which encode polymorphic transplantation antigens in the development and function of the immune system. Several upstream regulatory elements are shown to control expression of MHC class I genes. We isolated several cDNA clones encoding proteins that bind to the regulatory elements of MHC class I genes. These genes are conserved throughout the animal kingdom, have distinct DNA binding domains, and bind to regulatory elements of many genes. Their multiple binding specificity suggests a combinatorial mechanism of gene regulation. Thus interaction of multiple factors that are shared by multiple genes may determine how a target gene is transcribed. We now study functions of cloned DNA binding proteins. We have constructed plasmids that express the cloned DNA binding proteins in mammalian tissue culture cells. The following is a summary of our study on H-2RIIBP, a zinc finger protein that binds to the region II enhancer of MHC class I genes. Introduction of H-2RIIBP into undifferentiated embryonal carcinoma cells leads to enhancement of MHC class I promoter activity: this enhancement requires the DNA binding domain through which binding to the target gene occurs as well as the C-terminal domain to which unidentified ligand is expected to bind. We found that H-2RIIBP heterodimerizes with other hormone receptors, including the retinoic acid receptor and thyroid hormone receptor. Because H-2RIIBP heterodimers bind to target DNA elements at higher efficiency than homodimers, it is likely that H-2RIIBP functions by interacting with other members of hormone receptors. We study how various heterodimers regulate expression of target genes by using in vitro transcription assays. We now test the effect of recombinant H-2RIIBP on in vitro transcription. Functions of the cloned DNA binding proteins is being studied by in vivo footprinting analysis as well.